I vote for the 6...............There are "things" you could do to make the 235 more powerful, dual carbs, larger exhaust, lump port heads, larger bore, etc........Everybody goes for the V8 option, it's overdone. A built 6 would be cool and serve your needs. Also, you could go for a built 250 or 292.
My '60 Chevy with stock 235 keeps up on the highway just fine. 60 to 65 MPH easy all day. Even with 195 ft/lbs torque stock, it pulls hills with no problem. I wouldn't pull a camp trailer though without mod's.......
Try talking to the "Inliners", I think their website is
www.Inliners.org. They eat, sleep, drink inline engines.